By Thomas R. Collins
October 7, 2021
Article In Brief
CRISPR-Cas9 gene editing knocked out the production of misfolded transthyretin, which slowed the formation of amyloid, a contributor to transthyretin amyloidosis with polyneuropathy. The study was small and short-term, but experts said the targeted treatment approach could help alleviate mild disease.
A CRISPR-based gene-editing therapy for patients with hereditary transthyretin amyloidosis (ATTR) with polyneuropathy dramatically reduced levels of the disease-causing, misfolded transthyretin protein, according to findings published in the August 5 issue of the New England Journal of Medicine.
The results of the phase 1 study open-label trial showed the therapy was safe, raising the possibility of a more effective and more appealing treatment for the progressive and fatal disease, in which amyloid builds up in the body’s organs and tissues, where it can cause loss of sensation, heart problems, or other symptoms. Life expectancy after diagnosis is about seven to 12 years. Current treatment involves long-term therapy to stabilize the protein and slow the formation of amyloid or inhibit TTR protein production by degrading its mRNA, thereby slowing the formation of amyloid.
The new therapy, called NTLA-2001, was given once to six patients at 0.1 mg/kg to half and 0.3 mg/kg to the other half. After 28 days, the concentration of TTR protein in the serum was reduced by 52 percent in those receiving the lower dose and by 87 percent in those receiving the higher dose.
“Systemic administration of NTLA-2001 to six patients with hATTR amyloidosis with polyneuropathy was associated with dose-dependent reduction in the serum TTR protein concentration,” said the researchers, led by Julian Gillmore, MD, PhD, head of the Centre for Amyloidosis & Acute Phase Proteins at the National Amyloidosis Center at University College London.
How the Therapy Works
The therapy involves delivery of a single guide RNA molecule that specifically targets the gene that encodes transthyretin and mRNA that encodes the Streptococcus pyogenes Cas9 protein, which ‘edits’ the target gene. These are packaged in a lipid nanoparticle that is delivered intravenously. After it enters the circulation, it is primed for delivery to the liver by apolipoprotein E.
Once in the hepatocyte, the single-guide RNA and Cas9 mRNA are released, and they interact to form a ribonucleoprotein complex that is imported into the nucleus. There, it accesses the DNA helix, and cleaves the relevant TTR sequence.
Researchers said they identified seven potential off-target editing sites, but in laboratory studies with DNA from the whole genome and in human hepatocytes, they found no evidence of off-target editing at these sites at relevant concentrations of the drug, Dr. Gillmore and colleagues said. The investigators observed few adverse events in the clinical trial and all were mild.
They also acknowledged the early stage of the research. “It is important to note that this study involves a very small number of patients with limited follow-up to date,” they said. “Continued serial measurements of serum TTR concentration in the patients reported here are planned to confirm the durability of the effect.”
Data from existing agents that target RNA, requiring ongoing treatment, show that the lower the concentration of TTR that’s achieved with the therapy, the better the clinical results. That’s cause for optimism for this CRISPR-based treatment, they suggested.
“On the basis of data in animals, NTLA-2001 may be able to produce nearly complete and permanent knockdown of TTR expression with a single administration,” researchers said.
They said they are continuing to try to optimize the treatment. “Dose escalation continues with a goal of producing greater reductions in serum TTR protein than are achieved with available therapies, with anticipated beneficial effects on disease progression, quality of life, and mortality,” they wrote.
Michael J. Polydefkis, MD, FAAN, professor of neurology and director of the Cutaneous Nerve Lab at Johns Hopkins, called the findings “revolutionary” but emphasized the early nature of the research.
“It builds on the foundation of other cutting-edge therapies that knock down TTR production at the mRNA level—patisiran and inotersen,” he said. Those drugs have “transformed a progressive, disabling and often fatal disease into an eminently treatable one. It’s been remarkable to observe that. These latest results show that TTR knockdown at the genetic level also works and could be more sustained. Instead of receiving treatment every week or every few months, it could be one treatment.”
But he said the questions of off-target effects and the effects of the reduction in TTR protein need to be explored further. CRISPR Cas9 therapy in sickle cell anemia and other diseases has so far not been shown to have off-target effects, and that, he said, “builds confidence.” But, he added, “Six patients for one month is not enough experience” with this new therapy.
With other therapies, reduction in TTR protein—which, among other things, transports the thyroid hormone thyroxine—has been safe, given the excess TTR typically available in the body, but the effects of a higher reduction over a longer period of time are uncertain, he said.
“We do not know what would happen if TTR production were knocked down 100 percent,” he said. “This protein has a known physiological function and permanent, dramatic knockdown off TTR could result in unexpected complications. If it is truly a one-time treatment, we need to be sure the dose is correct the first time—there won’t be any mulligans.”
Still, one simplified delivery could be a huge improvement for patients if the treatment pans out, he said. “That’s certainly an improvement. It allows people to not think about the disease and kind of get on with their lives.”
Colin Chalk, MDCM, associate professor of neurology and neurosurgery at McGill University, said the findings show how powerful it can be to have precise knowledge of the molecular mechanism of a disease, given how finely targeted this therapy is.
The results so far suggest the treatment will likely have clinical effects in certain patients. “The reduction in serum TTR levels, particularly in the higher-dose patients, is impressive,” Dr. Chalk said. “Follow-up beyond 28 days is needed to see if the TTR reduction is sustained and to ensure that there are no unanticipated consequences of a nearly complete removal of normal TTR from patients. If these follow-up concerns can be dealt with, one assumes that the treatment would be most effective in patients with mild, early disease. The clinical efficacy remains to be proven, although it seems reasonable to expect that there will be clinical efficacy. A single, long-lasting treatment would have clear advantages over currently available therapies.”
Dr. Gillmore has served on the advisory board and received speaker fees from Alnylam Pharmaceuticals. He has also served on the advisory board of Eidos Therapeutics Inc., Intellia Therapeutics, and Ionis Pharmaceuticals Inc.
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